This invention relates generally to fiber optic sensors. In particular, this invention relates to apparatus and methods for adjusting the lengths of optical fibers used in unbalanced fiber optic interferometric sensor systems.
Optical fibers are sensitive to a large number of physical phenomena, such as acoustic waves and temperature fluctuations. An optical fiber exposed to such phenomena changes the amplitude, phase or polarization of light guided by the fiber. Optical fibers have been considered for use as sensing elements in devices such as hydrophones, magnetometers, accelerometers and electric current sensors.
Mach-Zehnder and Michelson interferomaters respond to the phenomenon being sensed by producing phase differences in interfering light waves. Detecting phase changes in the waves permits quantitative measurements to be made on the physical quantity being monitored.
Mach-Zehnder interferometers are particularly sensitive to acoustic vibrations. A fiber optic Mach-Zehnder interferometer typically has a reference arm comprising a first length of optical fiber and a sensing arm comprising a second length of optical fiber. The sensing arm guides a sensing signal, and the reference arm guides a reference signal. These signal combine in an optical coupler to produce an interference pattern that depends upon the optical path difference between the sensing and reference arms. The sensing arm is exposed to a physical parameter, such as an acoustic wavefront, to be measured while the reference arm is isolated from changes in the parameter. When the Mach-Zehnder interferometer is used as an acoustic sensor, acoustic wavefronts change the optical length of the sensing arm as a function of the acoustic wave intensity. An optical coupler divides a light signal between the two arms. Changes in the length of the sensing arm are indicated by changes in the phase of the sensing and reference signals. The signals are combined after they have propagated through the reference and sensing arms, and the phase difference of the signals is monitored. Since the signals in the reference and sensing arms had a definite phase relation when they were introduced into the arms, changes in the phase difference are indicative of changes in the physical parameter to which the sensing arm was exposed.
A Michelson interferometer also has a sensing arm and a reference arm that propagate sensing and reference signals, respectively. However, in the Michelson interferometer these arms terminate in mirrors that cause the sensing and reference signals to traverse their respective optical paths twice before being combined to produce an interference pattern.
The performance of a dual wavelength, mismatched pathlength interferometric sensor relies on matching the frequency difference between two optical pulses used to interrogate the sensor to the optical path difference between the sensing and reference arms of the sensor. This frequency difference may be expressed mathematically as EQU f=c/(4nL) (1)
where
f=frequency difference between optical pulses; PA1 c=free space velocity of light; PA1 n=effective refractive index of the optical fiber; and PA1 L=length difference between interferometer arms.
For a single sensor, one can tune frequency shifting circuitry to adjust the optical signal input frequency to meet the conditions of Equation (1). However, when the same pair of optical pulses interrogate multiple sensors, as for example in a time division multiplexed system, each sensor must be well-matched to the next. If the path differences are not properly matched, then it may be difficult or impossible to match sensors and their outputs. Errors in path difference produce optical phase noise in the sensor system. In order to achieve the required matching between sensors, the error in path difference between the compensating interferometer and the sensing interferometer must be well within the coherence length of the optical signal source in the system.